Lightweight structures for use in the aircraft and spacecraft technologies include an outer skin which is strengthened on its inner surface by a reinforcement or stiffening framework. Such a framework is in fact a three dimensional structure but may be referred to as being “two-dimensional” to distinguish the framework from a three-dimensional aircraft or spacecraft body. An aircraft body, for example, is constructed as a lightweight structure with an outer skin that is reinforced by longitudinally extending stringers and circumferentially extending ribs. The stringers and ribs are adhesively bonded to the inner surface of the outer skin. In designing such lightweight structures, particularly in the aircraft industry, a special value is allocated to reducing weight while simultaneously keeping in mind, depending on the particular type of use, the strength requirement, the fatigue requirement, and the tolerance against damages, referred to herein as damage tolerance. Further, lightweight structures used in the aircraft construction must meet special requirements regarding the damage tolerance that lightweight aircraft components must have for safety reasons.
Increasing the damage tolerance of such lightweight structures can be accomplished in different ways. One example involves increasing the entire skin thickness, or providing different skin thicknesses in different locations throughout the lightweight structure so that the skin is thicker in locations exposed to higher loads while the skin is thinner in locations exposed to lesser loads. Strengthening the skin by increasing the thickness of the skin even only locally, increases the overall weight more than is acceptable. Another possibility of increasing the skin strength resides in using materials which themselves have an improved damage tolerance. Such materials are disclosed in German Patent Publication DE 102 38 460 A1 and counterpart U.S. Pat. No. 7,100,871, which describes metallic laminated materials or fiber composite laminates which are on the market under the Trademark GLARE®.
Fiber reinforced metal laminates have the advantage of a very good damage tolerance combined with a relatively small density compared to monolithic metallic sheet materials. However, presently fiber reinforced metal laminates have a lower static strength characteristic compared to monolithic sheet metal materials. As a result, a weight reduction of the lightweight structure is only possible in a few areas where the particular component is primarily designed with regard to damage tolerant characteristics rather than strength characteristics. Furthermore, the production of fiber reinforced metal laminates is relatively expensive compared to the production of monolithic sheet metal materials due to the, at least partially, complicated pretreatment which the metal sheets require for the subsequent adhesive bonding operation and due to the use of additional prepreg films in the metal laminates and the need for manually positioning and preparing the individual laminae for the subsequent curing and adhesive bonding operation. As a result, the production costs can be significantly higher than the production costs of monolithic sheet metals. Significantly smaller costs are involved in the production of metallic laminate materials without fiber reinforcement as described in the above mentioned German Patent Publication DE 102 38 460 A1.